<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .big{
            width: 300px;
            height: 300px;
            background-color: greenyellow;
            position: relative;
            margin: 0 auto;
        }
        .little{
            width: 100px;
            height: 100px;
            background-color: orange;
            position:absolute;
            left:  50%;
            margin-left: -50px;
        }

    </style>
</head>
<body>
    <div class="big">
        <div class="little"></div>
    </div>
</body>
</html>